The several time-bound effects of prestimulation on the human startle reflex offer the possibility that different levels of neural processing may be distinguished and may elucidate the nature of changes occurring in early development. The first approach will be to determine the age at which effects ascribable to a short-time processing system can be demonstrated. This will involve manipulating duration, rise time, and spectral composition of startle and of lead stimuli and measuring the effects on primary and secondary components of startle. The second approach will be to determine whether facilitatory effects, presumed to depend on the classical activation system mediated via brain core mechanisms, are present at birth and are modulated with increasing age and cortical maturation. The third approach will investigate the development of effects presumed to depend on orienting-attentional processes.